Selecting a charger and charging batteries

Most deep-cycle applications have a charging system already installed for battery charging (e.g. solar panels, inverters, golf car chargers, alternators, etc.). However, if the original battery is being replaced with a different technology or a new battery charger is needed, care should be taken to choose the right charger.

When changing between Flooded, AGM and GEL-type batteries, the existing charger may need to be changed and/or the voltage cut-offs reset. Many chargers installed by original equipment manufacturers have selector switches built into their systems, allowing the user to change between Flooded, AGM and GEL-type batteries.

What follows will help in making a proper selection.

There are many types of chargers available today. They are usually rated by their voltage and initial or starting rates, or the rate in amps that the charger will supply at the beginning of the charge cycle. When selecting a charger, the charge rate should be between 15% and 30% of the battery’s 20-hour AH capacity. For example, a battery with a 20-hour capacity rating of 100 AH should use a charger rated between approximately 15 and 30 amps (for multiple battery charging, use the total AH rating of the entire bank to determine the charger rating required). The higher the initial charging current, the better. Take care not to exceed the manufacturer's recommended maximum. The faster the battery can be charged and left to rest before its subsequent discharge, the better.

We recommend using a charger from one of the high-quality charger manufacturers Discover has approved. OEM chargers with Discover-approved charge curves prolong battery life and enhance performance.

Approved OEM charge curves for Discover Dry Cell AGM batteries:


Alternatively, we recommend using a 3-stage voltage compensating and temperature sensing charger with a generic charge curve approved for AGM batteries. These chargers usually have three distinct charging stages: Bulk, Absorption and Balance or Float stage.


Approved generic charge curves for Discover Dry Cell AGM batteries:


Some chargers that claim to have an equalize mode or stage should not be used on AGM or GEL batteries. Discover-approved chargers have “Balance Charge” modes in their “single cycle” charge algorithms. Balance Charging is similar to an equalize charge, but it is performed using specific temperature-adjusted current, voltage and time limits. ALWAYS ATTEMPT TO SIZE YOUR CHARGING SOURCE TO CHARGE THE BATTERY WITHIN 10 HOURS IF POSSIBLE. Give the battery time to rest before starting a new discharge.

Approved OEM charge curves for Discover Dry Cell GEL batteries:


Alternatively, we recommend using a 3-stage voltage compensating and temperature-sensing charger with a generic charge curve approved for GEL batteries. These chargers usually have three distinct charging stages: Bulk, Absorption and Balance or Float stage.

Approved generic charge curves for Discover Dry Cell GEL batteries:


Can I equalize an AGM or GEL battery?

DO NOT EQUALIZE CHARGE AGM or GEL BATTERIES! Equalizing is an “over voltage-over charge” performed on flooded lead-acid batteries after they have been fully charged to help eliminate acid stratification. It helps eliminate acid stratification and sulfation in all flooded lead-acid batteries. Acid Stratification is the #1 killer of flooded lead-acid batteries. Laboratory-designed, specific to Discover, “Balance” charge algorithms have been designed and are included in all Discover recommended Industrial Commercial charges offered for sale wherever our Discover AGM and GEL VRLA DRY CELL products are sold. Visually the Balance Charging curve diagram is similar to a flooded equalize charge curve in its finishing voltage. Still, it is strictly controlled to promote proper active material conversion and not to promote gassing or bubbling of the acid to help reduce acid stratification as in a flooded battery. Discover’s Balance charging algorithms are HIGHLY recommended for batteries installed in strings, larger batteries using taller plate groups and/or batteries constantly being subjected to high-rate deep discharges.

How do I know if a charger is “Gel friendly” or “AGM friendly”?

Unfortunately, many chargers on the market claim to be Gel “friendly” or “OK for sealed batteries” but are not. Some overcharge the batteries, while others may not fully charge the batteries. Some chargers claim to be “smart.” Some “smart” chargers do a good job, while others do not. The best choice of charger often depends on the application. Use only “voltage-regulated” or “voltage-limited” chargers if possible. Standard constant current or taper current chargers must not be used. Almost all applications require temperature sensing and voltage compensation. Beware, many chargers measure the ambient temperature, which could be significantly different from the battery’s internal temperature. Be sure that if using a non-temperature compensated charger, the charger's profile and charging “time-outs” recognize this!

Low-frequency current ripple (to about 333Hz) can be detrimental to sealed batteries depending on the application. In applications where the charger is connected continuously to a float voltage, especially where simultaneous charge and discharge may occur, the current ripple level must be considered.

If you are not sure if a charger is performing correctly, follow this procedure:

  • Using a fully discharged VRLA DRY CELL battery (OCV about 11.8V) and a digital voltmeter, record the initial open circuit voltage at the battery terminals.
  • Using an automatic charger described above, set voltage if adjustable (14.2V for GEL, 14.5V for AGM models).
  • Connect and start charging—record initial on-charge voltage and current.
  • Check and record the on-charge voltage across the battery terminals each hour or so. Except for occasional, brief “blips” or pulses, the voltage should not exceed the voltage limits noted in “b” above.
  • At the end of the charge (when the current is very low or goes to zero), check and record the voltage. Note that the charger may have turned off by then.
  • The disconnected battery should be at 100% or above after a 24-hour rest.

During the charging time, the charger should not have exceeded the limit (except for occasional, brief pulses). This indicates that the charger is working correctly. Keep in mind that the voltage limit is 77˚F/25˚C. Charging at higher or lower temperatures will change this limit.

A temperature-sensing charger should always be used when charging VRLA DRY CELL batteries, as manual adjustments are never accurate and exceeding temperature limits will damage any battery. Discover has worked with many quality charger manufacturers to develop a complete line of chargers specifically designed for its Discover products, with the capability to charge all other types of lead-acid batteries as well. Please enquire with a Discover dealer!